That will work only as long as you can simultaneously keep pressure on his forearm with your right arm and make the jabs at his eyes.
The problem I have with this is twofold. First, it allows the BG to get the gun out of the holster. This is bad for any number of reasons, not the least of which is the danger to bystanders, your feet/legs, side, etc. For if the BG is able to wrench his arm free, you are potentially DOA.
Secondly, in martial arts it is well known that concentration on our muscle groups can lapse when the mind is preoccupied with other things. For example, if someone has you in a rear choke hold, slamming your heel down on his instep might work to injure him, but the toes are more sensitive and more easily broken. Slamming onto them, even if it doesn't cause pain, distracts the mind from the hand you have on their wrist. If you pull on their wrist just after slamming onto their foot you can usually gain a few inches to duck out of the choke hold. Thus, in the above scenario, YOUR mind is diverted while poking at his eyes and he may be able to break your forearm "grip" at that point.
One method that I did see demonstrated could be used if you were alert to someone getting "too close" (instead of being surprised). Push your strong hand down on the gun & his hand then TURN into him with your off-hand palm up and rigid. Strike to the diaphram area with a straight jab as HARD as you can, trying to twist your off-side shoulder into the movement. When hit, his grip weakens and you can peel it off the gun and twist away. If done correctly you won't have to shoot him because he will be preoccupied just trying to breathe!. I should add that striking the throat will work too, but increases the likelihood of a crippling or fatal injury. The heel of the hand to the chin or base of the nose could work too, if you have the reach.
BillCA in CA (Unfortunately)